1. Field of the Invention
The present invention relates first of all to a method for controlling a thermal printing or writing head of series type for a printing system, particularly for a printer connected to a word processing device.
2. Description of the Prior Art
A thermal writing head generally includes a plurality of heating elements, generally resistive elements adapted so as to have a current flowing therethrough, and to cooperate either directly with a printing medium, for example a thermosensitive paper, or indirectly with an ordinary printing medium, through a ribbon coated with an ink melting under the heat of the elements. In the first case, it is a question of direct thermal printing and in the second case thermal transfer printing.
The invention relates to series type thermal printing and, in this case, the mobile head has at least one generally vertical strip of elements, aligned in a direction orthogonal to the direction of rectilinear movement, during the printing of a line of graphics corresponding to the height of a strip, of a carriage supporting the head.
More particularly, the invention relates to a method for controlling a series type thermal writing head for a printer having a strip of heating elements, in which:
said head is driven with a longitudinal movement along a line to be printed, at a given speed,
the heating elements being subjected to thermal cycles, each including an initial heating time and a cooling time, for printing dots.
In this method, the assembly of heating elements which are heated during the same cycle depends obviously on the configuration of the cross section of the line to be printed, the dimension of which is L, equal to the product of the moving speed multiplied by the duration T of the cycles.
In such a method, and taking for example the extremely frequent case in which the line is written horizontally, the vertical definition depends solely on the size and spacing of the heating elements of the writing head. On the other hand, the horizontal definition is approximately inversely proportional to the speed of driving the head. In fact, taking into account the thermal inertia of a heating element, it is not possible to make the time interval as small as desired during which it is hot enough for printing. Since this time interval is then defined by a lower limit, the horizontal dimension of the "dot" which it prints is therefore substantially inversely proportional to the speed of driving the head.
In the above defined method, a compromise must then be found between the horizontal definition and the writing speed of the head.
The aim of the present invention is to obtain a better compromise, that is to say better horizontal definition for a constant writing speed or a higher writing speed for equivalent horizontal definition.